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This thesis presents numerical investigations of two-dimensional single-phase turbulent jets and bubbly mixing layers using Reynolds-Averaged Navier-Stokes (RANS) equations. The behavior of a turbulent jet confined in a channel depends on the Reynolds number and geometry of the channel which is given by the expansion ratio (channel width to jet thickness) and offset ratio (eccentricity of the jet entrance). Steady solutions to the RANS equations for a two-dimensional turbulent jet injected in the middle of a channel have been obtained. When no entrainment from the channel base is allowed, the flow is asymmetric for a wide range of expansion ratio at high Reynolds number. The jet attaches to one of the channel side walls. The attachment length increases linearly with the channel width for fixed value of Reynolds number. The attachment length is also found to be independent of the (turbulent) jet Reynolds number for fixed expansion ratio. By simulating half of the channel and imposing symmetry, we can construct a steady symmetric solution to the RANS equations. This implies that there are possibly two solutions to the steady RANS equations, one is symmetric but unstable, and the other solution is asymmetric (the jet attaches to one of the side walls) but stable. A symmetric solution is also obtained if entrainment from jet exit plane is permitted. Fearn et al. (Journal of Fluid Mechanics, vol. 121, 1990) studied the laminar problem, and showed that the flow asymmetry of a symmetric expansion arises at a symmetry-breaking bifurcation as the jet Reynolds number is increased from zero. In the present study the Reynolds number is high and the jet is turbulent. Therefore, a symmetry-breaking bifurcation parameter might be the level of entrainment or expansion ratio. The two-dimensional turbulent bubbly mixing layer, which is a multiphase problem, is investigated using RANS based models. Available experimental data show that the spreading rate of turbulent bubbly mixing layers is greater than that of the corresponding single phase flow. The presence of bubbles also increases the turbulence level. The global structure of the flow proved to be sensitive to the void fraction. The present RANS simulations predict this behavior, but different turbulence models give different spreading rates. There is a significant difference in turbulence kinetic energy between numerical predictions and experimental data. The models tested include 𝘬—𝜖, shear-stress transport (SST), and Reynolds stress transport (SSG) models. All tested turbulence models under predict the spreading rate of the bubbly mixing layer, even though they accurately predict the spreading rate for single phase flow. The best predictions are obtained by using SST model. / Master of Science

No / Objective The number of colonoscopies required to reach competency is not well established. The primary aim of this study was to determine the number of colonoscopies trainees need to perform to attain competency, defined by a caecal intubation rate (CIR) ≥90%. As competency depends on completion, we also investigated trainee factors that were associated with colonoscopy completion. Design The Joint Advisory Group on GI Endoscopy in the UK has developed a trainee e-portfolio from which colonoscopy data were retrieved. Inclusion criteria were all trainees who had performed a total of ≥20 colonoscopies and had performed ≤50 colonoscopies prior to submission of data to the e-portfolio. The primary outcome measure was colonoscopy completion. The number of colonoscopies required to achieve CIR ≥90% was calculated by the moving average method and learning curve cumulative summation (LC-Cusum) analysis. To determine factors which determine colonoscopy completion, a mixed effect logistic regression model was developed which allowed for nesting of patients within trainees and nesting of patients within hospitals, with various patient, trainee and training factors entered as fixed effects. Results 297 trainees undertook 36 730 colonoscopies. By moving average analysis, the cohort of trainees reached a CIR of 90% at 233 procedures. By LC-Cusum analysis, 41% of trainees were competent after 200 procedures. Of the trainee factors, the number of colonoscopies, intensity of training and previous flexible sigmoidoscopy experience were significant factors associated with colonoscopy completion. Conclusions This is the largest study to date investigating the number of procedures required to achieve competency in colonoscopy. The current training certification benchmark in the UK of 200 procedures does not appear to be an inappropriate minimum requirement. The LC-Cusum chart provides real time feedback on individual learning curves for trainees. The association of training intensity and flexible sigmoidoscopy experience with colonoscopy completion could be exploited in training programmes.

The evolution of column instabilities that lead to break up of a microscale liquid jet is studied experimentally using shadowgraph technique. The jet formation is investigated over a range of Reynolds number, Pressure Ratio, and Ohnesorge number which are varied by the driving pressure, observation chamber pressure, and the jet liquid. Over the range of these parameters, the jet experiences different break up mechanisms as a result of different dominant instabilities. The present investigation discusses both break up mechanisms that are similar to the break up of macroscale jets and some new microscale break up phenomena.

La trempe à l’air est largement utilisée dans les procédés de production de verre de sécurité. L’obtention d’une distribution de contraintes adéquate requiert un refroidissement intense et homogène à la fois, et ces deux propriétés sont difficiles à obtenir sur la courte durée de la trempe. Les batteries de jets utilisées dans la plupart des systèmes de trempe produisent un refroidissement adéquat mais souffrent d’inhomogénéité, à l’origine de défauts de trempe et de casse durant le processus.L’objectif de cette thèse est d’explorer des nouvelles configurations qui améliorent l’homogénéité du refroidissement en préservant son intensité. L’approche choisie consiste à implanter des jets rotatifs dans les réseaux de manière à accentuer le mélange des jets avant impact. Les études ont été menées principalement par simulation numérique, corroborées par des visualisations par enduit gras sur un banc d’essai dédié, conçu et réalisé dans le cadre de cette thèse.La première phase a été consacrée à la conception des générateurs de jets rotatifs et à l’étude de leur dynamique en mode isolé. Le développement d’une structure tourbillonnaire se formant à l’entrée de chaque lobe du dispositif de mise en rotation a été mis en évidence. L’interaction des jets rotatifs dans le réseau de refroidissement constitue la deuxième phase. Il apparait que la structure cellulaire du schéma d’impact n’est que marginalement perturbée par les jets rotatifs et que la présence de ces derniers n’influe que peu sur la dynamique de l’écoulement. Enfin, la modélisation détaillée des transferts de chaleur sur la plaque d’impact montre que les jets rotatifs ne contribuent que faiblement au refroidissement, mais que l’interférence avec le réseau de jets simples augmente légèrement le transfert de chaleur local au niveau de leur impact. Sans avoir obtenu les résultats escomptés, cette thèse a toutefois montré la complexité du système et le couplage fort entre les phases d’alimentation et d’évacuation de l’air de refroidissement. / Air quenching is widely applied in security glass manufacturing processes. Proper residual stresses distribution requires strong and homogeneous cooling and both are difficult to achieve over the very short time of the tempering process. Jet arrays used in most processes provide with sufficient cooling but suffer from inherent inhomogeneity, leading to quality loss of the glass product and, in extreme cases, to unacceptable breaking numbers during production.The objective of the present study is to investigate ways to improve cooling homogeneity while maintaining efficiency. For this purpose, swirling jets are located inside the jet arrays to enhance jet mixing prior to impingement. Numerical simulation is performed, corroborated by oil flow visualization and a dedicated test bench has been designed and set up within the frame of this thesis.The first part was concerned with the design of swirlers and their dynamic behaviour in standalone mode. It has been shown that a vortex is forming at the inlet of each swirl compartment. Inserting the swirlers within jet arrays constitutes the seconf phase. It turns out that the cellular structure of the impingement pattern is only marginally affected by the swirlers, which have a weak influence on the flow dynamics. Last, the detailed heat transfer modeling on the impingement surface shows that the swirlers themselves do barely contribute to the overall cooling, while the coupling with the simple jet array slightly improves the local heat transfer close to the impingement area. Although the expected outcome was not achieved, this thesis showed the flow complexity as well as the strong coupling between the feeding and the exhaust phases experienced by the cooling air.